More than 50％ of the world’s population now lives in cities. The urban population will reach 1 billionby 2030 in China.Continuing urban warming has been observed and studied in many megacities in the world. InHong Kong, for example, the temperature has increased by 0.17oC per decade since 1947, which is faster than theaverage global warming rate. Urban warming has led to increased mortality due to heat stress. A 1oC increase inthe mean air temperature in Hong Kong increases its electricity use by 4.5％. At the same time, The air in HongKong has become more and more stagnant, with, for example, wind speed in Kowloon reducing by 0.6 m/s perdecade since 1968 and 0.35 m/s per decade since 1995.At least two fundamental differences exist in urban winds between high-rise compact and low-rise sparse cities.First, wind can easily penetrate into a low-rise sparse city, but it cannot penetrate into a high-rise compact city dueto the large canopy drag. Calculation using the existing theory shows that the wind speed at street level in somecompact locations in Hong Kong is less than 0.05％ of the wind speed above. Second, the buoyancy driven flowsalong building walls (wall flows) become dominant in high-rise compact canopy layers as the wind aloft inducedflow is weak and because of the building heights and large total wall areas, as shown by our study. We also findthat analyzing changes in the characteristics of the daily temperature cycle (mean, amplitude, phase, max/min) dueto urban anthropogenic factors is the key to understanding urban warming dynamics. In Hong Kong over the last100 years, the time lag of the daily max air temperature behind solar radiation has shifted from 1 pm to 2:30 pm.Similar changes in the daily time lag can also be observed in major cities in the US and China since the 1950s, butthe delay by 1.5 hours in Hong Kong is the largest found so far. Anthropogenic factors are more significant inhigh-rise compact cities than in others.We hope that our new understanding provides new insights into how a city can reduce its warming trend andguide urban planning/design not only for Hong Kong but other cities in the world where urbanisation is rapidlyincreasing.